1. Field of the Invention
The present invention relates to a wastewater treatment apparatus and method for removing nitrogen and phosphorus, and more particularly, to a wastewater treatment apparatus and method, the wastewater treatment apparatus having an anaerobic tank, an anoxic tank, an aerobic tank and a clarifier, wherein the aerobic tank has a baffle installed at one side thereof to form a dissolved oxygen reducing zone for reducing the concentration of dissolved oxygen contained in internally recycled wastewater returned from a dissolved oxygen reducing zone while increasing the concentration of dissolved oxygen contained in treated effluent supplied from a part other than the dissolved oxygen reducing zone of the aerobic tank to a clarifier in a subsequent stage.
2. Description of the Related Art
Nitrogen components in wastewater exist in forms of organic nitrogen and inorganic nitrogen, which are termed total nitrogen (T-N). The inorganic nitrogen is classified into ammonia nitrogen and nitrate nitrogen. The organic nitrogen and the ammonia nitrogen are termed total Kjeldahl nitrogen (TKN). Most of nitrogen in wastewater is measured as TKN. In order to biologically remove nitrogen components contained in wastewater, conversion into nitrate nitrogen (NOX) (nitrification) must be preceded. The influent nitrogen in wastewater is released to the air in the form of gaseous N2 converted by denitrification after being converted into nitrate nitrogen through nitration by microorganisms. The denitrification requires presence of organic matter and requires no dissolved oxygen (DO) present in the wastewater.
In order to remove phosphorus from wastewater, a phosphorus release reaction must be accomplished by microorganisms under anaerobic conditions and then the released phosphorus are taken up by the microorganisms using oxygen or oxygen in nitrate nitrogen. Thus, phosphorus can be removed from wastewater while increasing intercellular phosphorus of microorganisms. For effectively releasing phosphorus using microorganisms, the concentration of nitrate nitrogen must be low.
As described above, the nitrification for removal of phosphorus requires abundant dissolved oxygen, while the denitrification or phosphorus release reaction requires no dissolved oxygen present. The microorganisms accomplishing denitrification or removal of phosphorus are heterotrophic bacteria requiring an organic carbon source as an energy source. Theoretically, organic matter with a COD concentration of 2.86 g is required in removing 1 g of nitrate nitrogen, and organic matter with a COD concentration of 40 g is required in removing 1 g of phosphorus.
Denitrification is largely divided into a reaction occurring in the case where organic matter is present and a reaction occurring in the case where organic matter is absent. The reaction for the latter case is called endogenous denitrification, which is performed at a low speed, requiring a longer retention time. On the other hand, in the case where organic matter is present, denitrification is performed at a very high speed, thereby reducing a retention time. Also, the denitrification rate may differ according to the kind of organic matter.
There are two types of phosphorus removing microorganisms; one is phosphorus accumulating organisms (PAO) which accomplish a bacterial metabolism using oxygen in anaerobic and aerobic tanks; and the other is denitrifying phosphorus accumulating organisms (dPAO) which accomplish a bacterial metabolism using oxygen contained in nitrate nitrogen. The use of oxygen increases oxygen demand and activates the synthesis of cells of microorganisms in the anaerobic tank, resulting in increases in repair and maintenance costs. On the other hand, the use of oxygen contained in nitrate nitrogen simultaneously causes phosphorus uptake and denitrification under anaerobic conditions, increasing nitrogen and phosphorus removal efficiency and suppressing synthesis of cells of microorganisms, thereby reducing the repair and maintenance costs.
Biological removal of nitrogen and phosphorus requires organic matter for denitrification and release of phosphorus under anaerobic conditions. In this case, costs for chemicals can be reduced by using the required organic matter from raw wastewater to be treated, compared to the case of using external carbon source. Also, the nitrogen and phosphorus removal efficiency may vary according to the concentration of organic matter contained in raw wastewater and the amount of influent wastewater.
In most conventional wastewater treatment methods, nitrogen and phosphorus have been removed using an organic carbon source contained in influent wastewater, and if necessary, phosphorus has been removed using chemicals. In order to maintain a reaction tank at a predetermined state, even the same biological removal process must employ different techniques depending on the position of return sludge. Also, most conventional wastewater treatment methods are based on the assumption that phosphorus removing microorganisms cannot accomplish denitrification.
One example of conventional wastewater treatment methods for removing nitrogen and phosphorus is disclosed in U.S. Pat. No. 4,867,883 to Daigger et al., as shown in FIG. 1.
Wastewater 10 primarily passed through a clarifier is introduced to an anaerobic tank 101 together with wastewater 12a returned from the output end of an anoxic tank 102. In the anaerobic tank 101, a phosphorus release reaction by microorganisms occurs using organic matter present in influent wastewater under the condition that there is no dissolved oxygen.
Wastewater 11 having undergone the phosphorus release reaction in the anaerobic tank 101 is introduced to the anoxic tank 102 together with return sludge 15a and wastewater 13a returned from an oxic tank 103. In the anoxic tank 102, denitrification of nitrate nitrogen present in the wastewater 13a returned from the oxic tank 103 occurs using remaining organic matter under the condition that there is no dissolved oxygen.
Wastewater 12 passed through the anoxic tank 102 is introduced to the oxic tank 103, and nitrification and luxury uptake of phosphorus occur, removing organic matter.
The wastewater passed through the oxic tank 103 is subjected to solid-liquid separation so that supernatant is discharged as treated water 14 and some of settled sludge 15a is returned to the anoxic tank 102 and the remainder 15b is wasted.
Another conventional wastewater treatment process for removing nitrogen and phosphorus is disclosed in U.S. Pat. No. 4,056,465 to Spector et al., as shown in FIG. 2.
The wastewater treatment process of Spector et al. is different from that of Daigger et al. in that return sludge 25a supplied from a clarifier 204 is introduced to an aerobic tank 201, rather than to an anoxic tank 202, and that some of wastewater 22 passed through the anoxic tank 202 is not returned to the anaerobic tank 201. Thus, in the anaerobic tank 201, a phosphorus release reaction is carried out under the condition that no nitrate nitrogen nor dissolved oxygen are present.
In both wastewater treatment processes according to Daigger et al. and Spector et al., wastewater to be treated is introduced to an anaerobic tank to cause a phosphorus release reaction by microorganisms or both a phosphorus release reaction and denitrification and then to cause denitrification in an anoxic tank. Thus, in the case where the content of organic matter is low, like in sewage, a predetermined amount of organic carbon source contained in influent wastewater is consumed up in an anaerobic tank. Thus, it is difficult for denitrification to smoothly take place in an anoxic tank that follows, which means that it is necessary to provide external carbon source. Also, efficiencies of removing nitrogen and phosphorus become different, thereby unavoidably extending the overall retention time in the reaction tank.
Also, denitrification efficiencies of the anoxic tank may be different depending on the amount of nitrate nitrogen in the wastewater returned from the output end of the oxic tank to the anoxic tank or in the return sludge. Thus, nitrogen or phosphorus in wastewater cannot be removed in a stable manner. Also, there is a limitation in removing high-concentration nitrogen and phosphorus.
Another conventional wastewater treatment process for removing nitrogen and phosphorus, which is called a Bardenpho process, is disclosed in U.S. Pat. No. 3,964,998, as shown in FIG. 3.
Primarily clarified wastewater 30 and return sludge 35a supplied from a clarifier 304 are introduced to an anaerobic tank 301. In the anaerobic tank 301, a phosphorus release reaction by microorganisms is carried out using organic matter in influent wastewater under the condition that no nitrate nitrogen nor dissolved oxygen are present.
Internally recycled wastewater 31b returned from a first aerobic tank 303a and the wastewater 31 passed through the aerobic tank 301 are introduced to a first anoxic tank 302a, and denitrification occurs in the first anoxic tank 302a using remaining organic matter under the condition that there is no dissolved oxygen.
Wastewater 32 treated by the first anoxic tank 302a is introduced to the first aerobic tank 303a, and nitrification and luxury uptake of phosphorus take place in the first aerobic tank 303a. Some of the wastewater 31b passed through the first aerobic tank 303a is introduced to the first anoxic tank 302a to be internally recycled, and the remainder 31a is introduced to a second anoxic tank 302b so that nitrogen is removed by endogenous denitrification by microorganisms.
Wastewater 32a treated by the second anoxic tank 302b is introduced to a second aerobic tank 303b and increases the deaerating action and settleability of microorganisms in a settlement site.
Wastewater 33 passed through the second aerobic tank 303b is introduced to a clarifier 304 and solid-liquid separation occurs thereat. Some of the sludge 35 settled in the clarifier 304 is returned to the anaerobic tank 301 as return sludge 35a and some is removed as waste sludge 35b. 
While the wastewater treatment process of Barnard advantageously increases the efficiency of denitrification and increases sludge settleability by microorganisms by removing nitrogen based on endogenous denitrification by further providing the second anoxic tank 302b and the second aerobic tank 303b to the wastewater treatment process of Spector et al. However, further providing the second anoxic tank 302b and the second aerobic tank 303b undesirably resulted in increases in time and cost required for treatment.
Still another conventional wastewater treatment apparatus and method for removing nitrogen and phosphorus is disclosed in Korean Patent Publication No. 2001-087698, as shown in FIG. 4.
The wastewater treatment apparatus includes an anaerobic tank 401, an anoxic tank 402, an oxic tank 403 and a clarifier 404. Sludge settled in the clarifier 404 is returned to the oxic tank 403. A dissolved oxygen removal tank 403a is installed along a return line from the oxic tank 403 to the anoxic tank 402. Raw wastewater 40 to be treated is introduced into the anaerobic tank 401. The wastewater 41 passed through the anaerobic tank 401 is fed to the anoxic tank 402. The wastewater 42 passed through the anoxic tank 402 is fed to the oxic tank 403. Some wastewater 43a passed through the oxic tank 403 is returned to the anoxic tank 402 for removing nitrogen and the remainder 43 is fed to the clarifier 404. Denitrification occurs in the anoxic tank 402.
Sludge is settled in the clarifier 404, and purified water is discharged outside as treated water 44. Some sludge 45 is again fed to the oxic tank 403. Water 43a is returned from the oxic tank 403 to the anoxic tank 402 via the dissolved oxygen removal tank 403 to remove dissolved oxygen contained in the returned water, thereby increasing the processibility of the anoxic tank 402.
Although the above-described treatment process increases the denitrification efficiency of an anoxic tank by allowing treated water to be returned from an aerobic tank to the anoxic tank via a dissolved oxygen removal tank, the necessity of additionally installing the dissolved oxygen removal tank is disadvantageous from the viewpoints of cost and time.